In 2017, the J. R. Dahn team used three in-situ characterization methods to analyze the changes in volume, stress, and thickness of the silicon-containing negative electrode cells of three different positive and negative electrode systems.Combined with the calculation model, the expansion mechanism of the three system cells is further given, which has certain guiding significance for the design and application of the cell.
Type A: Li(Ni1-x-yCoxAly)O2 (NCA)/SiO-graphite
Type B: LiCoO2 (LCO)/Si Alloy-graphite
Type C: Li(Ni1-x-yCoxAly)O2(NCA)/nano Si-C
In-situ test method
1. In-situ volume expansion test: Archimedes's law of buoyancy, test the buoyancy change of the battery cell in silicone oil, as shown in Figure 1.
Figure 1. Schematic diagram of volume expansion test
2. In-situ stress expansion test: real-time monitoring of stress changes during charging and discharging by fixing a pressure sensor on the surface of the cell, as shown in Figure 2a) and 2b).
3. In-situ thickness expansion test: The displacement sensor monitors the up and down movement of the aluminum plate on the surface of the battery cell to monitor the thickness change during charging and discharging, as shown in Figure 2c).
Figure 2. Schematic diagram of stress and thickness expansion testing
1.The in-situ volume, stress, and thickness change curves of the three system cells
Comparing the charge-discharge curve with the volume, stress, and thickness change curve, it can be seen that they all increase during charging and decrease during discharge.This is mainly due to the fact that the volume expansion of the negative electrode with lithium insertion during charging is much greater than the volume contraction of the positive electrode with lithium removal, so the net expansion is increased, but the opposite during discharge.When the A and C cells are close to the fully charged state, the three expansion curves have a flat trend, while the B cell shows a sharp transition trend.
Figure 3. In-situ volume, stress, and thickness change curves of the three systems
2.Positive and negative electrode expansion analysis
Based on previous literature studies, when the battery is charged, the volume expansion of the Si negative electrode is greater than 280%, the volume expansion of graphite is about 10%, the volume expansion of LCO is about 1%, and the volume contraction of NCA is about 1%.Therefore, it is speculated that the reason for the flat volume change of the cells A and C when they are close to the fully charged state is that the volume contraction of the positive electrode NCA offsets part of the expansion of the negative electrode, so the overall volume of the cell does not change much.
Figure 4. Percentage of volume change of four materials when lithium is released
3.Quantitative calculation of positive and negative electrode expansion of cell A
The author uses dV/dQ analysis and fitting software to split the voltage curve of the silicon-carbon negative electrode during the charging and discharging process of cell A.As shown in Figure 5.Combined with the individual expansion curves of the positive and negative materials, the total volume expansion curve of the cell A can be further split, and there are three stages of changes:i) The platform is graphite 2L→2 phase transition;ii) There are two mechanisms of action in the stage:The shrinkage of NCA offsets the expansion of graphite and no volume shrinkage during the phase transition of graphite 2→2L;The iii) stage is mainly due to the rapid volume shrinkage caused by the delithiation reaction of the lithium-silicon alloy in SiO.
Figure 5. Voltage curve fitting of silicon carbon anode
Figure 6. Quantitative disassembly of the expansion curve of cell A
4.Comparison of long-cycle stress curves of batteries B and C
As the cycle time increases, it can be clearly found that the capacity attenuation and stress increase of cell B are greater than that of cell C.It shows that the cathode material of the NCA system can inhibit the cyclic expansion of the battery cell more than the cathode material of the LCO system, and thus has a better capacity retention rate.
Figure 7. Long-cycle stress curves of batteries B and C
In this paper, three in-situ characterization methods are used to analyze the changes in volume, stress and thickness of the silicon-containing negative electrode cells of three different positive and negative electrode systems.Combined with the calculation model, the expansion mechanism of the three system cells is further given, which has certain guiding significance for the design and application of the cell.
SWE series in-situ expansion analysis system (IEST Yuanneng Technology):
1.Integrate a variety of in-situ characterization methods of the cell (stress & expansion thickness): simultaneously measure the expansion thickness and expansion force of the battery during charging and discharging, and quantify the change of the expansion thickness and expansion force of the battery;
2. A more precise and stable test system: the use of a highly stable and reliable automatic adjustment platform, equipped with a high-precision thickness measurement sensor and a pressure adjustment system, to achieve long-term monitoring of the battery's long-term charging and discharging process;
3. Diversified environmental control and testing functions: SWE series equipment can adjust the temperature of the charging and discharging environment to help the study of cell expansion behavior under high and low temperature conditions;
A. J. Louli, Jing Li, S. Trussler, Christopher R. Fell, and J. R. Dahn. Volume, Pressure and Thickness Evolution of Li-Ion Pouch Cells with Silicon-Composite Negative Electrodes. Journal of the Electrochemical Society, 164 (12) A2689-A2696 (2017)